Currently there is a problem with the server.properties on bedrock servers where you cannot edit it and it gives you a 404 error, however there is still a way to do edit it:
Locate your files tab and scroll down to server.properties, if you do not see it, start your server and switch it off again for it generate.
This is what server.properties should look like:

Now when we right click the image or press the three dots on the right hand side, we will be able to select an option called "Rename", select this option and put .zip at the end, this is what it should look like after you save your changes:

Now you are able to go into your server.properties and edit it. Once you are done, you must rename the file so that it goes back to its original name, server.properties
There are some things that will not change and you have to change from a separate menu.
Here is the list:
- Server name
- Gamemode
- Difficulty
- Allow cheats
You can edit these from the Startup tab which should look like this:

That is all you need to know when editing your server.properties file on your bedrock server! (Make sure you save your changes by the way!)